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\r^ I no>in IIIIIIH^^^^^ BUREAU OF MINES ^^ 

lU 9240 IIH^ INFORMATION CIRCULAR/1990 



i {^/lAR -• 8 199(J I 




Methods for the Analysis of Mineral 
Chromites and Ferrochrome Slag 



By D. A. Baker and J. W. Siple 




BUREAU OF MINES 
1910-1990 

JHE MINERALS SOURCE 



'U OF 



Mission: Asthe Nation's principal conservation 
agency, the Department of the Interior has respon- 
sibility for most of our nationally-owned public 
lands and natural and cultural resources. This 
includes fostering wise useof our land and water 
resources, protecting our fish and wildlife, pre- 
serving the environmental and cultural values of 
our national parks and historical places, and pro- 
viding for the enjoyment of life through outdoor 
recreation. The Department assesses our energy 
and mineral resources and works to assure that 
their development is in the best interests of all 
our people. The Department also promotes the 
goals of the Take Pride in America campaign by 
encouraging stewardship and citizen responsibil- 
ity for the public lands and promoting citizen par- 
ticipation in their care. The Department also has 
a major responsibility for American Indian reser- 
vation communities and for people who live in 
Island Territories under U.S. Administration. 



Information Circular 9240 



Methods for the Analysis of Mineral 
Chromites and Ferrochrome Slag 

By D. A. Baker and J. W. Siple 



UNITED STATES DEPARTMENT OF THE INTERIOR 

Manuel Lujan, Jr., Secretary 

BUREAU OF MINES 
T S Ary, Director 






Library of Congress Cataloging in Publication Data: 



Baker, D. A. (Delbert A.) 

Methods for the analysis of mineral chromites and ferrochrome slag / by D. A. 
Baker and J. W. Siple. 

p. cm. - (Information circular / Bureau of Mines; 9240) 

Includes bibliographical references. 

Supt. of Docs, no.: I 28.27:9240. 

1. Chromium ores-Analysis. I. Siple, J. W. II. Title. III. Series: Information 
circular (United. Bureau of Mines); 9240 

~-4:N295.U4 [TN490.C4] 622 s-dc20 [553.4'643'0287] 89-600322 

CIP 



I 



CONTENTS 

Page 

Abstract 1 

Introduction 2 

Total chromium in mineral chromite and ferrochrome slags 2 

Acid-soluble or "metallic" chromium in ferrochrome slags 4 

Total iron in mineral chromite and ferrochrome slags 6 

Ferrous iron in miner2d chromite and ferrochrome slags 8 

MetaUic iron in ferrochrome slags 9 

Aluminum and magnesium in mineral chromite and ferrochrome slags 10 

Calcium in mineral chromite and ferrochrome slags 11 

Gravimetric sihca in mineral chromite and ferrochrome slags 12 

Manganese in mineral chromite, ferrochrome slags, zmd ferrochrome 14 

Acid digestion of ferrochrome metal 17 

Method A 17 

Method B 17 

Discussion 18 

References 18 

Appendix A.-Reagent preparation 19 

Appendix B— Use of zirconium crucibles 21 

Appendix C.-Notes on sample preparation 22 



UNIT OF MEASURE ABBREVIATIONS USED IN THIS REPORT 


°c 


degree Celsius 


mL 


milliliter 


g 


gram 


N 


normal concentration. 


h 


hour 


nm 


nanometer 


L 


liter 


pet 


percent 


meq wt 


milliequivalent weight 


ppm 


part per million 


mg 


milligram 


s 


second 


min 


minute 







METHODS FOR THE ANALYSIS OF MINERAL 
CHROMITES AND FERROCHROME SLAG 



By D. A. Baker^ and J. W. Siple^ 



ABSTRACT 

This report describes the elemental characterization of chromite and related materials at the Bureau 
of Mines, Albany Research Center. Analytical methods for determining the major constituents, 
representing extensive experience, refinement, and development, are described and fully annotated. This 
presentation should allow other laboratories to use these methods and obtain comparable results with 
a minimum of time needed for famiUarization with individual methods. 



^Chemist. 

Albany Research Center, Bureau of Mines, Albany, OR. 



INTRODUCTION 



A goal of the Bureau of Mines is the maintenance of an 
adequate supply of critical minerals and metals to meet 
national economic and strategic needs. As part of this 
effort, the Bureau's Albany Research Center has con- 
ducted research related to chromium ores and production. 
In providing analytical support, the Center has analyzed 
many samples of chromium materials and has acquired 
considerable experience in the requisite techniques. 

Chromium is on the critical materials hst because it is 
widely used in strengthening and enhancing the corrosion 
resistance of ferrous alloys and there is a lack of adequate 
domestic resources. In general, the United States has 
relied on imports to supply 91 pet of its chromium needs, 
depending on secondary recovery to supply the remaining 
9 pet (ly 

Research projects at the Albany Research Center have 
ranged from the location and characterization of domestic 
sources of chromite ore, through smelting ferrochrome 
ingot operations, to the recovery of chromium and 
chromium-bearing minerals from various industrial wastes. 
Reports published include Bureau of Mines Information 
Circular 8916, entitled "Podiform Chromite Occurrences 
in the Caribou Mountain and Lower Kanuti River Areas, 
Central Alaska. Part II: Beneficiation" (2) and Bureau of 
Mines Report of Investigations 8676, entitled "Chromium 
Recovery From Nickel-Cobalt Laterite and Laterite Leach 
Residues" (3). 

In support of these and other research projects, this 
Center has analyzed a large number of samples of 
chromite ore, ferrochrome, ferrochrome slag, and solids 
and solutions from chromium recovery investigations. The 



large number of samples processed has promoted refine- 
ment of the analytical methods used, so that reliability of 
the results is not sacrificed in order to return the data in 
a timely manner. The results of this refinement are 
presented in this publication. 

The purpose of this report is to present the techniques 
used at the Albany Research Center for the analysis of 
chromite minerals and ferrochrome-related samples. 
The methods given in this report are based on standard 
techniques {4-13). The presentation is concise and offers 
supplementary information to aid the understanding of less 
experienced analysts and technicians. Toward this goal, 
the format for the presentation of each method includes: 

1. A short introductory note on each method. 

2. Lists of equipment and materials. 

3. A step-by-step listing of the procedure. 

4. Procedure notes that provide information pertaining 
to the reasons for a step in the method or supplementary 
instructions referring to indications of success, malfunction, 
or optional actions that may be taken. 

The preparation of reagent solutions is presented in 
appendix A. Appendixes B and C present some notes on 
the use of zirconium crucibles and on sample preparation. 

This format should allow the experienced analyst to 
take this report into the laboratory and produce results 
that are comparable and compatible with results obtained 
at this Center, with a minimum of delay for familiarization 
with the techniques, and this format should accelerate the 
understanding of the less experienced analyst. 



TOTAL CHROMIUM IN MINERAL CHROMITE AND FERROCHROME SLAGS 



The total chromium content in mineral chromite and 
ferrochrome slags can range from a fraction of a percent 
in the slags to greater than 30 pet in the ore concentrates. 
Because of this variation and because of the difficulty in 
dissolving chromite-containing samples, the method of 
choice at this Center has been fusion with sodium 



peroxide, followed by persulfate oxidation and titration 
with ferrous iron. 

The wet method is sufficiently accurate and reproduc- 
ible for high-quality results on a routine basis. The sam- 
ples may be determined very rapidly without a sacrifice in 
quality. Batches of 20 determinations have been com- 
pleted in 6 h by one analyst without assistance. 



Equipment 

Zirconium crucible (approximately 30-mL capacity 
seems most convenient). 

Meker or similar gas-air mix burner. 

400-mL beaker and cover. 

Glass stirring rod. 

Hotplate. 

Rubber policeman. 

Burette. 



Italic numbers in parentheses refer to items in the Hst of references 
preceding the appendixes at the end of this ref)ort. 



Materials 

Sodium peroxide, reagent grade, granular, 20 mesh or 
finer. 

Sulfuric acid, reagent grade, diluted 1:1 with distilled 
water. 

Saturated manganese solution. 

Silver nitrate, reagent grade, 2.5-pct solution. 

Ammonium persulfate, reagent grade, crystal. 

Hydrochloric acid, reagent grade, concentrated. 

Phosphoric acid, reagent grade, concentrated. 

Ferrous iron solution, standardized. 

Sodium diphenylamine sulfonate solution. 



Procedure 



1. Weigh the sample into a zirconium crucible. 

2. Add 4 to 12 g of sodium peroxide, and stir imtil the 
peroxide and sample are thoroughly mixed. 

3. Fuse over a burner imtil all sample particles are dis- 
solved. Swirl and inspect occasionally to keep unattacked 
particles dispersed. 

4. When the fusion is complete, allow the crucible and 
melt to cool. 

5. When they are cool, tap gently to free the soHdified 
melt from the bottom of the crucible. 



11. To the solution in the beaker, add one drop of a sat- 
urated manganese solution, 2 or 3 mL of a 2.5-pct solu- 
tion of silver nitrate, and 3 to 6 g of ammonium persulfate. 
Stir thoroughly. 

12. Bring the solution to a boil, and boil for several min- 
utes to decompose the excess persulfate. 

13. Remove the beaker from the hotplate, and immedi- 
ately add 5 mL concentrated hydrochloric acid to decom- 
pose the permanganate. 

14. Stir and cool the solution to room temperature or 
below. 



6. Place the solidified melt and the emptied crucible in 
a 400-mL beaker, add 150 to 200 mL of cold distilled wa- 
ter, and cover quickly. 

7. When leaching activity has subsided, slide the cover 
aside slightly and slowly add about 25 mL of 1:1 sulfuric 
acid solution. Reclose the cover and wait for the com- 
pletion of any further leaching action. 

8. Remove and rinse the cover glass, and rinse down the 
beaker sides. 

9. Place a glass stirring rod in the beaker and stir the 
contents thoroughly. 

10. Remove and thoroughly rinse the crucible. Inspect 
the inside of the crucible and poUce it if necessary. 



15. When the solution is cool, add about 10 mL of con- 
centrated phosphoric acid, and titrate with a standard fer- 
rous iron solution using two to five drops of sodium di- 
phenylamine sulfonate solution as the indicator, which 
changes from purple to green at the endpoint. 

Titration equation: 

Cr+^ + 3Fe^^-* Cr+^ + SFe'^l 



Calculation: 



+ 2 



mL titer x N Fe x eq wt Cr 



sample wt x 1,000 



1 mL O.IOOOA^ Fe 



+ 2 



X 100 = pet total Cr. 



1.733 mg Cr. 



Procedure Notes 



1. Sample size is usually roughly calculated to yield a 
titration of about 20 to 50 mL (as a convenience). How- 
ever, a minimum of 0.2 g is used when adequate sample is 
available. A maximum of about 1 g is used to keep the 
melt volume to a manageable level. 

2. Increase the sodium peroxide to match sample size 
using an approximate ratio of 20 parts of peroxide to 
1 part of sample. Again, consideration must be given to 
melt volume at high sample weights. Thorough mixing 
cannot be overstressed. Sample particles in the melt can 
aggregate and stubbornly resist attack. 

3. The burner should be capable of bringing the crucible 
to red heat on the bottom. Mixing remains important. If 
particles are allowed to aggregate, fusing time may have to 
be extended, which increases the attack on the crucible 
itself, shortening its useful life, increasing the tendency of 
the cooled melt to stick to the crucible, and adding a sig- 
nificant concentration of zirconium to the analyte solution. 



4. If the melt is to be leached quickly, the crucible can 
be set out on any heat-resistant material to cool. If the 
melt has to be set aside for a while, it should be set on a 
hotplate on low heat to keep it from absorbing atmo- 
spheric water (which makes the melt stick to the crucible). 

5-6. If gentle tapping does not free the melt, try moder- 
ately hard tapping. If the melt still sticks, lay the crucible 
on its side in the bottom of the beaker. 

7. The leaching reaction can range from very slow to 
very vigorous. A high aluminum concentration tends to 
slow the reaction. A slow leach reaction may become a 
very fast leach reaction when sulfuric acid is added, so care 
must be exercised. Once the leach reaction has finished, 
the addition of sulfuric acid is generally accompanied by 
moderate effervescence unless a significant amount of car- 
bonate is present. 



8-10. Rinse down any visible precipitate particles and 
police out any residue in the crucible. Stirring should pro- 
duce a clear solution unless hydrolyzed zirconium or alu- 
minum is present. Do not add more sulfuric acid unless 
iron or chromium hydroxide precipitate is present. Excess 
sulfuric acid may slow or prevent the oxidation of 
chromium. 

11. The manganese is to indicate the completion of the 
chromium oxidation. AH of the chromium will be oxidized 
before the manganese begins to be converted to perman- 
ganate. If permanganate is the salt added, it may not all 
reduce and may leave a permanganate pink. Proceed re- 
gardless. The permanganate color after the decomposition 
of the excess persulfate is the important condition, not the 
pink in the intermediate stages. The silver nitrate is a 
catalyst in the oxidation reaction. Crystalline ammonium 
persulfate is used to save the time and labor of preparing 
fresh solution. 

12. When the oxidation is complete and the manganese 
turns red, it turns quickly, in 1 or 2 s. As the excess per- 
sulfate is destroyed, a mild effervescence can be seen in 
the solution. If the solution does not turn red after boiling 
for a few minutes, remove it from the hotplate and inspect 
it for precipitate that looks Hke silver chloride. If found, 



add more silver nitrate and cimmonium persulfate, and re- 
turn the solution to the hotplate. If no such precipitate is 
found, just add more ammonium persulfate, and return the 
solution to the hotplate. Repeat the operations as 
necessary. Lost water may be replaced without any hazard 
to results. 

13. The chloride in the hydrochloric acid will be oxidized 
to chlorine gas in the process of reducing the permanga- 
nate. It will also precipitate the silver. The chloride will 
not react with the dichromate produced. 

14. Be aware that a small amount of chlorine gas is 
evolved during cooling. 

15. The phosphoric acid is added to complex the ferric 
iron. If significant amounts of aluminum or titanium are 
present, hydrofluoric acid may be substituted for the phos- 
phoric acid. The ferrous iron solution is prepared from 
ferrous ammonium sulfate in 12-L batches. A piece of 
purified aluminum sheet kept in the carboy helps to keep 
the normality of the solution constant. Sodium diphenyl- 
amine sulfonate is used because it is easily soluble in dis- 
tilled water and thus preparation is simplified. Any of the 
diphenylamine indicators will work, giving a sharp endpoint 
by changing from purple to green. 



ACID-SOLUBLE OR "METALLIC" CHROMIUM IN FERROCHROME SLAGS 



At this Center, results from this method have been used 
almost exclusively as data for the calculation of the total 
degree of reduction in a smelter batch, rather than as an 
indicator for the metallization of chromium. When a 
charge of chromite material is smelted or prereduced, the 
crystal structure is changed, and once the chromite spinel 
structure has been opened, the chromium becomes much 
more available to acid attack. Partially reduced chromium 

Equipment 

250-mL beaker. 
400-mL beaker. 
Stirring rod. 
Hotplate. 
Filter paper. 
Funnel. 
Burette. 



species and chromium in the form of silicates, carbonates, 
or other carbon compounds, as well as chromium metal, 
are then available to acid attack. (These species of chro- 
mium are often grouped under the name "metallic" chro- 
mium.) (It has been noted that a very finely ground sam- 
ple of a mineral chromite concentrate will generally give 
up a few relative percent of its chromium to this method.) 



Materials 

Sulfuric acid, reagent grade, diluted 1:1 with distilled 
water. 

Hydrofluoric acid, 46 to 51 pet, reagent grade. 
Saturated manganese solution. 
Silver nitrate solution. 

Ammonium persulfate, reagent grade, crystal. 
Hydrochloric acid, reagent grade, concentrated. 
Phosphoric acid, reagent grade, concentrated. 
Ferrous iron solution, standardized. 
Sodium diphenylamine sulfonate solution. 



Procedure 



1. Weigh the sample into a 250-mL beaker. 

2. Add 80 to 90 mL of distilled water, 10 mL of 1:1 sul- 
furic acid, and 5 to 10 mL of hydrofluoric acid. 

3. Place the beaker on a hotplate and bring to a gentle 
boil or just below. 

4. Digest at a gentle boU, or just below, for 40 to 
50 min, replacing lost water whenever the digestion 
approaches half the original volume. 

5. Remove the solution from the hotplate, cool to about 
room temperature, and filter through a medium-speed 
qualitative paper (such as Schleicher and Schuel"* (S&S) 
597) into a 400-mL beaker. Wash 5 to 10 times with dis- 
tilled water. 

6. Dilute the combined filtrate and washes up to 200 to 
250 mL total volume. 

7. Add one drop of saturated manganese solution, 2 or 
3 mL of 2.5-pct silver nitrate solution, and 3 to 6 g of 
ammonium persulfate, rinse sides, and stir thoroughly. 

8. Bring the solution to a boil, and boil several minutes 
to destroy the excess persulfate. 



9. Remove the beaker from the hotplate, and imme- 
diately add 3 to 5 mL of concentrated hydrochloric acid to 
decompose the permanganate. 

10. Stir thoroughly, rinse sides, and cool to room tem- 
perature or below. 

11. When the solution is cool, add about 10 mL of con- 
centrated phosphoric acid emd titrate with a standard fer- 
rous iron solution using three to five drops of sodium di- 
phenylamine sulfonate solution as the indicator, which 
changes from purple to green at the endpoint. 

Titration equation: 

Cr + ^ + 3Fe + 2 -> Cr"^^ + 3Fe + l 



Calculation: 



+ 2 



mL titer x A^ Fe x eq wt Cr 



X 100 



sample wt x 1,000 
pet acid-soluble Cr. 

1 mL O.IOOOA^ Fe^^ = 1.733 mg Cr. 



Procedure Notes 



1. Sample size is usually roughly calculated to yield a 
convenient titration. Sample sizes from a few hundred 
milligrams for prer educed smelter charges up to several 
grams for slag are easily handled, though large samples of 
finely ground material sometimes have a tendency to 
bump. 

2. The described acid mixture is proper for all normal 
determinations. Another small volume of hydrofluoric acid 
may be added if high silica or metal passivation is sus- 
pected. Do not add more sulfuric acid. A high sulfuric 
acid concentration can slow or prevent the oxidation of 
chromium from Cr*^ to Cr^*. 

3-4. Gentle digestion is preferred over vigorous boiling 
because it requires much less attention and entails less 
danger of passivating any metallics present. 

5. Very fine gangue particles will sometimes pass 
through ("slime" through) the paper, but they do no harm 
in the oxidation or titration steps, other than making the 
solution cloudy. 



4 

Reference to specific products does not imply endorsement by 
the Bureau of Mines. 



6. Aim at 150 to 200 mL solution volume at the start of 
the titration. 

7. Saturated manganese solution is to indicate the 
completion of the chromium oxidation. All of the chro- 
mium will be oxidized before the manganese is converted 
to permanganate. The silver nitrate is a catalyst in the 
chromium oxidation. Crystalline ammonium persulfate is 
used to save the time and labor of preparing fresh 
solution. 

8. When the oxidation is complete and the manganese 
turns red, it turns quickly. As the excess persulfate is de- 
stroyed, a mild effervescence can be seen in the solution. 
If the solution does not turn red after boiling for a few 
minutes, remove it from the hotplate and inspect it for 
precipitate that looks like silver chloride. If found, add 
more silver nitrate and ammonium persulfate, and return 
the solution to the hotplate. If no such precipitate is 
found, just add more persulfate and return the solution to 
the hotplate. Repeat these operations as necessary. Lost 
water may be replaced without any hazard to results. 

9. The chloride in the hydrochloric acid will be oxidized 
to chlorine gas in the process of reducing the 
permanganate. It will also precipitate the silver. The 
chloride will not react with the dichromate. 



10. Be aware that a small amount of chlorine gas is 
evolved during cooling. 

11. The phosphoric acid is used as a complexing agent for 
the ferric iron present. If significant amounts of titanium 
or aluminum are present, hydrofluoric acid may be 
substituted for the phosphoric acid. The ferrous iron 
solution is prepared from ferrous ammonium sulfate in 



12-L batches. A piece of purified aluminum sheet kept in 
the carboy helps to maintain constant normality of the 
solution. Sodium diphenylamine sulfonate is used because 
it is easily soluble in distilled water and thus preparation 
is simplified. Any of the diphenylamine indicators will 
work, giving a sharp endpoint by changing from purple 
to green. 



TOTAL IRON IN MINERAL CHROMITE AND FERROCHROME SLAGS 



Iron is the second major element in chromite. In smelt- 
ing operations where ferrochrome is the product, a specific 
range in the chromium-to-iron ratio is desired. Where 
pure metalUc chromium is the product, the iron concen- 
tration is not as important. In both cases, it is important 
to have a good value for the chromium-to-iron ratio. A 
typical chromite or chromite concentrate can range from 
less than 10 pet iron to over 20 pet iron. 

Equipment 

Zirconium crucible (approximately 30-mL capacity). 

Meker or similar gas-air mix burner. 

400-mL beaker and cover. 

Glass stirring rod. 

500-mL Erlenmeyer flask. 

Hotplate. 

Funnel. 

Filter paper. 

Burette. 



To cope with this variability and with the difficulty in 
attacking chromite samples, the method of choice at this 
Center is titration after alkaline fusion. This method is 
also used on ferrochrome slags, even though they may oc- 
casionally contain less than 1 pet iron. 

While this method is not as fast as the total chromium 
method, rapid and reliable results are easily obtained. 



Materials 

Sodium peroxide, reagent grade, granular, 20 mesh or 
fmer. 

Hydrochloric acid, reagent grade, concentrated. 

Aqueous ammonia, reagent grade, concentrated. 

Hydrochloric acid, reagent grade, diluted 1:1 with 
distilled water. 

Stannous chloride, reagent grade, 10-pct solution in 20- 
pct hydrochloric acid. 

Saturated mercuric chloride solution. 

Phosphoric acid, reagent grade, concentrated. 

Sodium diphenylamine sulfonate solution. 

Potassium dichromate, reagent grade, 0.1/V solution in 
distilled water. 



Procedure 



1. Weigh the sample into a zirconium crucible. 

2. Add 4 to 12 g of sodium peroxide, and stir untU the 
peroxide and sample are thoroughly mixed. 

3. Fuse over a gas and air burner imtil all sample par- 
ticles are dissolved. Swirl and inspect occasionally to keep 
unattacked particles dispersed. 

4. When the fusion is complete, allow the crucible and 
melt to cool. 

5. When they are cool, tap gently to free the solidified 
melt from the bottom of the crucible. 

6. Place the solidified melt and the emptied crucible in 
a 400-mL beaker, add 150 to 200 mL of cold distilled wa- 
ter, and cover quickly. 



7. When leaching activity has subsided, remove and rinse 
the cover and wash down the beaker sides. 

8. Slowly, with stirring, add concentrated hydrochloric 
acid until all the precipitated iron dissolves. 

9. Remove and thoroughly rinse the crucible, and police 
it if necessary. 

10. Slowly, with stirring, add concentrated aqueous am- 
monia until a permanent iron precipitate is obtained; then 
add 20 to 25 mL excess. 

11. Bring the solution to a boil, and boil for a few min- 
utes to destroy any remaining peroxide and produce a 
more easily filterable precipitate. 



12. Remove the beaker from the hotplate, and rinse the 

sides. 

13. Filter the solution through a fast-speed, quahtative 
paper (such as S&S 595). Wash the beaker once and wash 
the precipitate three to five times with distilled water, and 
discard the filtrate and washes. 

14. Wash the precipitate back into its original beaker with 
distilled water. 

15. Wash the filter paper free of iron with alternating 
rinses of hot 1:1 hydrochloric acid and distilled water. 
Make sure any precipitate particles stuck on the beaker 
sides cire redissolved. 

16. Add 20 to 25 mL of concentrated hydrochloric acid, 
stir until all the precipitate dissolves, and transfer the so- 
lution to a 500-mL Erlenmeyer flask. 

17. Set the flask on a hotplate and bring it to a boil. 
Reduce the iron with dropwise additions of 10-pct stan- 
nous chloride solution with agitation until the solution is 



colorless, or light green if Cr^^ is present. Add three to 
five drops excess, and bring to a boil again. 

18. Cool the solution to room temperature or below. 

19. When the solution is cool, add 10 mL of a saturated 
mercuric chloride solution, and mix. Immediately add 7 to 
10 mL of concentrated phosphoric acid and three to five 
drops of sodium diphenylamine sulfonate indicator, and 
titrate with standard potassium dichromate solution to the 
first permanent purple. 

Titration equation: 

3Fe^^ + Cr"^^ -* 3Fe^^ + Cr"^l 
Calculation: 

mL titer x A'^ K2Cr207 x eq wt Fe 



sample wt x 1,000 
pet total Fe. 



X 100 



+ 6 



1 mL O.lOOOyV Cr^° = 5.585 mg Fe. 



Procedure Notes 



1. Sample size is usually approximated to yield a 10- to 
30-mL titration. However, a minimum of about 0.2 g is 
used when adequate sample is available. A maximum of 
about 1 g is used to keep the melt volume to a manageable 
level. 

2. An approximate ratio of 20 pairts of peroxide to 1 part 
of sample is used. At high sample weights, resulting melt 
volume must be considered, however. The flux and sample 
must be thoroughly mixed, or the sample particles may 
aggregate and stubbornly resist attack. 

3. The burner should be capable of bringing the entire 
crucible bottom to red heat. Careful agitation must be 
done to keep the particles from aggregating. Extended 
fusion time increases the attack on the crucible itself and 
adds a significant concentration of zirconium to the analyte 
solution. It also tends to make the cooled melt stick to 
the crucible. 

4. If the melt is to be leached quickly, the crucible can 
be set out on any heat-resistant material to cool. If the 
melt has to be set aside for a while, it should be set on a 
hotplate at low heat to keep it from absorbing atmospheric 
water. 

5-6. If gently tapping does not free the melt, try mod- 
erately hard tapping.If the melt still sticks, lay the crucible 
on its side on the bottom of the beaker. 

7-8. The leaching reaction can range from very slow 
to very vigorous. A high iron concentration speeds the 



reaction. A high aluminum concentration tends to slow 
the reaction. A slow leach may become a very fast leach 
when acid is added. Once the leach is finished, the addi- 
tion of acid is generally accompanied by mild effervescence 
unless there is a considerable amount of carbonate present 
to evolve carbon dioxide. 

10. An cmimonia precipitation is done to free the iron of 
the large amount of sodium in the solution and to produce 
larger precipitate particles. A high sodium concentration 
has been found to depress the indicator change in the ti- 
tration. It has also been found that with a sodium hydrox- 
ide precipitation, a very fine-grained iron precipitate is 
produced. The coarser precipitate produced by the am- 
monia is easily filtered. A small excess of sodium peroxide 
added to the ammonia precipitation after step 10 will allow 
the precipitate to be freed of nearly all of the chromium 
present. Sodium peroxide is added 1 or 2 g at a time until 
a moderate effervescence is produced by gentle stirring 
and the beaker contents take on a deeper brown color. 
Chromium, usually present as hydrated chromic oxide 
(CrjOj) is oxidized to Cr*^ by the sodium peroxide and 
remains in the filtrate. This operation can be used to 
measure iron and chromium on the same sample but usu- 
ally needs to be repeated to remove all visible traces of 
chromium from the iron solution. If chromium is to be 
determined, the filtrate is acidified with 1:1 sulfuric acid 
and titrated as for total chromium. Results are slightly 
low, and the method is not recommended unless insuffi- 
cient sample is available for a separate determination. 



11-12. Bubbles from peroxide decomposition cause me- 
chanical problems in the filtration, often slowing it 
unnecessarily. 

13. It has been found convenient, after rinsing the beziker, 
to wash the sides once with hot 1:1 hydrochloric acid from 
a wash bottle. 

15. A glass wash bottle with insulation for holding is used 
to contain the 1:1 hydrochloric acid for washing. 

16. The Erlenmeyer flask is a matter of preference; the 
titration may be done just as well in the beaker. If a 
beaker is used, it should be covered during cooling. 

17. The stannous chloride reduces the Fe^^ to the Fe^^ 
form necessary for the titration. Interfering elements that 
are reducible by stannous chloride are rare in chromite 



mineral samples and ferrochrome slags. The excess 
staimous chloride protects the Fe^^ from air oxidation 
while cooling. 

19. Saturated mercuric chloride solution is added in 
excess to destroy the excess stannous ion; it does not 
otherwise participate in the titration reaction. Phosphoric 
acid is added as a complexing agent for Fe^^. If much 
titanium or zirconium is present in the solution, a gellike 
precipitate may form when the phosphoric acid is added. 
This interferes with mixing efficiency and requires a slower 
cmd more careful titration. It does not interfere chem- 
ically. Hydrofluoric acid may be substituted for the phos- 
phoric acid or added, in small amounts, in addition to the 
phosphoric acid to avoid this condition. The indicator is 
at first colorless, turns green as the endpoint is approached 
(very heird to see when much Cr*^ is also present), and 
sharply turns to purple at the endpoint. 



FERROUS IRON IN MINERAL CHROMITE AND FERROCHROME SLAGS 



The determination of ferrous iron in mineral chromites 
and other chromite-bearing samples is limited by the 
difficult solubility of the chromite lattice. Reaction of the 
ferrous iron immediately upon release from the crystal 
seems to be the preferred way of measurement. Attempts 
to dissolve chromite by other methods depend on strong 
oxidizing agents that leave no ferrous iron to be mea- 
sured. The V^''-V*^ system is stable enough to yield good 
results when it is used to react with released ferrous iron. 

The situation is complicated, however, by the lack 
of any standard materials with a certified ferrous iron 
value. National Bureau of Standards (NBS) 103a chrome 

Equipment 

150-mL beaker and cover glass. 

600-mL beaker. 

Stirring rod. 

Hotplate. 

Pipette (50-mL capacity). 

Burette. 



refractory, for example, Usts an FeO value that is total iron 
expressed as ferrous oxide. While this value should be 
fairly close, several relative percent of the ferrous iron are 
oxidized by atmospheric oxygen in grinding and storage. 

With the limitations in mind, researchers at this Center 
have used results from this method as data for the calcu- 
lation of smelter charges and of total reduction in a 
smelter run. 

The method requires an overnight digestion, but is 
otherwise simple and rapid. Reproducibility has been usu- 
cdly within 2 relative percent. 



Materials 

Vanadium-acid mixture. 

O.l/V Fe^^ standard solution. 

Sodium diphenylamine sulfonate solution. 



Procedure 



1. Weigh a sample to contain 1.5 meq vA Fe*^ or less 
into a 150-mL beaker. 

2. Add just enough distilled water to wet and disperse 
the sample. 

3. Pipette 50 mL of the vanadium-acid mix onto the 
sample while swirling the beaker to maintain dispersion. 
Pipette a blank for digestion with each batch of samples. 

4. Cover and digest overnight on a hotplate at about 
100° c. 



5. Remove from the hotplate and cool to room tem- 
perature or below. 

6. When the solution is cool, slowly pour it into a 
600-mL beaker containing 150 to 200 mL of water, while 
stirring. Thoroughly rinse the small beaker into the large 
beaker, and dilute the solution to about 500 mL total 
volume. 

7. Titrate with standard Fe^^ solution using three to five 
drops of sodium diphenylamine sulfonate indicator, which 
changes from purple to green at the endpoint. 



Titration equation: 



V+5 + Fe^2 



V+4 + Ft^\ 



Calculation: 

(mL blank titer - mL sample titer) x N Fe 



If metallic iron is present, it must be corrected for by 
subtracting three times the amount of iron present (in 
milliequivalents) from the milliequivalents of V^^ con- 
sumed, as in the following equation: 



. + 2 



. + 2 



= meq Fe in sample. 



. + 2 



(mL blank titer - mL sample titer) x N Fe 

- 3(meq Fe°) = meq Fe"*" . 
The result is then used in the second equation above. 



meq Fe x meq wt Fe , , 

— T-^ X 100 = pet Fe^ . 

sample wt 



Procedure Notes 



1. Complete solution is more reliably obtcdned when the 
sample is limited to 0.5 g or less. Fine grinding is essen- 
tial to the method, and all samples should be groimd to 
100 mesh or finer. 

2-3. It is necessary to give some attention to m containing 
the sample dispersion. The samples have a tendency to 
form lumps when the acid is added, and this negates the 
beneficial effects of fine grinding. Lumps of seunple stub- 
bornly resist attack. 



4. The acid mix wiU slowly attack glass. Thin or etched 
beakers should be avoided. 

5-6. Take care, this is a very strong acid solution. 

7. If metalhc iron is present it will consume 3 meq of 
V^^ solution for each milUequivalent of iron present. A 
separate metallic iron analysis is done, and the results are 
used to correct the Fe*^ results. 



METALLIC IRON IN FERROCHROME SLAGS 



It would truly be unusual to find a sample of mineral 
chromite with metallic iron in it. However, this Center has 
handled many samples of prereduced smelter charges with 
several percent of metallic iron and ferrochrome slags with 
measurable quantities of metallic iron. Therefore, this 
method finds its use in the later stages of mineral chromite 
processing. 

Equipment 

100-mL volumetric flask with screw cap. 

250-mL Erlenmeyer flask. 

150-mL beaker. 

Hotplate. 

Funnel. 

Filter paper. 

Pipette (50-mL capacity). 

Burette. 



Results from this method have been used to calculate 
total reduction in smelter runs, the efficiency of the parti- 
tion of metallics and slag materials, and the final smelter 
charge. 

The method is rapid and reliable and yields high-quality 
results on a routine basis. Reproducibility has typically 
been well within 1 relative percent. 

Materials 

Mercuric chloride, reagent grade, crystals. 
Hydrochloric acid, reagent grade, concentrated. 
Phosphoric acid, reagent grade, concentrated. 
Sodium diphenylamine sulfonate solution. 
0.1/V potassium dichromate solution, standardized. 



Procedure 



1. Weigh the sample into a 100-mL volumetric flask. 



4. Bring the solution to a boil, and boil gently for 1 min. 



2. Using nonmetaUic tools, add 8 to 10 g of mercuric 5. Remove the flask from the heat, and immediately 

chloride. screw the cap on snugly. 



3. Immediately add 40 to 50 mL of distilled water, and 
swirl the flask vigorously to mix the contents. 



6. Cool the solution to room temperature or below. 



10 



7. When the solution is cool, dilute to the mark with 
distilled water, reseal the flask, and mix the contents of 
the flask thoroughly. 

8. Filter into a dry beaker using a dry funnel and a dry, 
medium-speed qualitative paper (such as S&S 597). 

9 Pipette 50 mL of filtrate into a 250-mL Erlenmeyer 
flask. 



Titration equation: 



3Fe^^ + Cr+^ 



3Fe + ^ + Cr'^l 



Calculation: 

mL titer x A^ Cr"^^ x eq wt Fe x 2 



10. Add 5 to 10 mL of concentrated hydrochloric acid, 
5 to 10 mL of concentrated phosphoric acid, and three to 
five drops of sodium diphenylamine sulfonate indicator. 
Titrate with O.IN potassium dichromate solution to a 
purple endpoint. 

Procedure Notes 



sample wt x 1,000 
= pet metaUic Fe. 



xlOO 



1 mL O.IOOOA^ K2Cr207 



5.585 mg Fe. 



1. A maximum sample size of about 2 g is used to min- 
imize the error caused by soUd material in the volumetric 
flask. 

2. Metallic spatulas show definite signs of attack after 
contact with mercuric chloride; therefore, all handling 
should be done with glass, plastics, or porcelain utensils, 
and thorough caution should be used. The final solution 
after boiling should be saturated with mercuric chloride 
with a few excess crystals in evidence. 

3-6. The mercuric chloride begins attack upon contact 
with any metal. The attacked metal then becomes much 
more subject to air oxidation, so delays should be mini- 
mized until the solution is boiled and tightly stoppered. 



Chromium may be taken into solution aJso, but it does not 
interfere except by imparting a green color to the solution. 

7. Shake the flask until any materials caked on the bot- 
tom cu-e thoroughly dispersed. At this point, the flask may 
be left for up to 24 h before continuing. 

8-9. In summcuy, take a "dry ahquot" of 50 mL into a 
250-mL Erlenmeyer flask. Fine materials sometimes slime 
through the filter paper. They may be ignored unless the 
filtrate becomes too murky to see the endpoint. 

10. The hydrochloric acid provides the proper acid me- 
dium for the titration. The phosphoric acid complexes 
with ferric iron. The endpoint sharply turns to purple. 
The titrations should be carried out promptly. 



ALUMINIUM AND MAGNESIUM IN MINERAL CHROMITE AND FERROCHROME SLAGS 



Aluminum and magnesium occur as gangue components 
in chromite ores and ferrochrome slags. At this Center, 
ores from many different sites and of many varieties aie 
examined for commercial feasibility. The beneficiation of 
these ores is a continuing activity that produces most of 
the samples that are analyzed with this method. Data 
from this method are used in economic feasibility studies, 



Equipment 

Zirconium crucible (approximately 30-mL capacity). 

Meker or similau' gas-air mix burner. 

250-mL beaker 

Watchglass to fit 250-mL beaker. 

Stirring rod. 

100-mL volumetric flask. 

Assorted volumetric flasks. 

Hotplate. 

Rubber policeman. 

Assorted pipettes. 

AA spectrophotometer. 



to decide the best beneficiation methods for different ore 
types, and to cadculate the smelter charge. 

The method of choice at this Center is fusion with a 
minimum of sodium peroxide, solution with hydrochloric 
acid, and dilution to a proper range for determination by 
atomic absorption (AA) spectroscopy. This technique is 
rapid and accurate enough to handle a considerable 
sample load without sacrificing reliability. 

Materials 

Sodium peroxide, reagent grade, granular, 20 mesh or 
finer. 

Hydrochloric acid, reagent grade, concentrated. 
Appropriate AA standards. 



11 



Procedure 



1. Weigh 0.2 g of sample into a zirconium crucible. 

2. Add 3 to 4 g of sodium peroxide, and mix thoroughly. 

3. Fuse over a burner imtil all sample particles are 
dissolved. Swirl and inspect occasionally to keep unat- 
tacked particles dispersed. 

4. When the fusion is complete, allow the crucible and 
melt to cool. 

5. When they are cool, tap the crucible to dislodge the 
melt and place the melt in the 250-mL beaker. 

6. Add 10 to 20 mL of distilled water to the beaker, and 
cover quickly with the watchglass. 

7. Add about 5 mL of distilled water and 3 to 5 mL of 
concentrated hydrochloric acid to the crucible. 

8. When the leaching action has subsided in the beaker, 
uncover and rinse the cover and sides with a minimum of 
distilled water. 



9. Police the crucible, and pour the solution slowly into 
the beaker. Rinse the crucible carefully. 

10. Slowly, with stirring, acidify the leach in the beaker 
with concentrated hydrochloric acid; then add approxi- 
mately 5 mL excess. 

11. Place the beaker on a hotplate at low to medium heat 
for 15 to 30 min to clarify the solution. 

12. Remove the beaker from the hotplate, cool, and 
transfer the solution to a 100-mL volumetric flask. Make 
the flask up to the mark and mix. 

13. Make a 10:1 dilution of the sample solution for deter- 
mination of aluminum and a 100:1 dilution for determina- 
tion of the magnesium. 

14. Measure the absorbance of the aluminum at 309.3 nm 
and the magnesium at 285.2 nm. Set the other AA instru- 
ment parameters in accordance with the manufacturer's 
recommendations. 



Procedure Notes 



1. Experience has shown that 0.2 g is a convenient 
amount for nearly all chromite samples. 

2. The amount of sodium peroxide used needs to be the 
bare minimum necessary for rehable sample attack so that 
the AA instrument will not be overloaded with dissolved 

salts. 

3. The burner should be capable of bringing the crucible 
bottom to red heat. Mixing is important. If particles are 
allowed to aggregate, fusing time may have to be extended, 
increasing the attack on the crucible itself cind increasing 
the tendency of the cooled melt to stick to the crucible. 

4. If the melt is to be leached quickly, the crucible may 
be set on any heat-resistant surface to cool. If the crucible 
must be set aside awhile after cooling, it should be set on 
a hotplate on low heat to keep the melt from absorbing 
atmospheric moisture. Because of its small volume, the 
melt is usually easier to remove from the crucible if it has 
solidified with the crucible tilted. 



5-10. The small sample and melt size sometimes makes 
removal of the cooled melt difficult. The higher the iron 
content, the more vigorous the leach reaction. Police the 
inside of the crucible thoroughly. When finished, the so- 
lution volume should be less than 100 mL. 

11. If extended heating was necessary and the crucible 
attacked, the solution may not clarify because of the pres- 
ence of hydrolyzed zirconium. (This very rarely happens.) 

12. The solution may be left to evaporate if its final vol- 
ume is greater than 100 mL. 

13. A 10:1 dilution will yield a final dilution factor of 
1,000:1 for the aluminum. A 100:1 dilution will yield a fi- 
nal dilution factor of 10,000:1 for magnesium. Most sam- 
ples may then be determined using 10- and 20-ppm stan- 
dards for the aluminum and 2- and 4-ppm standards for 
the magnesium. 



CALCIUM IN MINERAL CHROMITE AND FERROCHROME SLAGS 



Calcium is rarely found in chromite ores in amounts 
greater than a few tenths of a percent, zmd calcium anad- 
ysis is rarely requested on ore samples. Calcium 
compounds are, however, used as slag conditioners for 
ferrochrome smelting. Most of the chromite-related 
samples received for calcium analysis at this Center have 



been slag samples. The data produced are used mainly to 
establish material balance in smelter calculations. 

The method of choice at this Center is fusion with a 
minimum of sodium peroxide, solution with hydrochloric 
acid, and dilution to a proper range for atomic absorption 
(AA) spectroscopy. 



12 



The method for the preparation of the sample for 
calcium analysis is identical to the preparation of the 
sample for aluminum and magnesium (previous section). 
Usually, the dilution prepared for aluminum is a proper 
concentration for calcium determination using 2- and 
5-ppm calcium standards. Occasionally, the dilution 
prepared for the magnesium determination is used when 
the calcium concentration is high. The undiluted solution 
(step 12 of the aluminum-magnesium method) is used 
when the calcium concentration is low. When analyzing 
for calcium, a reagent blank must be C8u-ried along to 



correct for the calcium content of the sodium peroxide. 
Better precision is obtained if the sodium peroxide flux is 
weighed when preparing the fusions for calcium analysis. 
The absorbance of the calcium solution is measured at 
211.3 nm with the other AA instrument parameters set 
according to the manufactxu'er's recommendations. When 
calcium determination is combined with the determination 
of fduminxmi and magnesium, samples may be run rapidly 
and large sample loads may be analyzed quickly without 
sacrificing reUability. 



GRAVIMETRIC SILICA IN MINERAL CHROMITE AND FERROCHROME SLAGS 



SiHca is present in chromite ores as a basic silicate. In 
characterization and beneficiation studies performed on 
chromites at this Center, the chromium-silica ratio 
determines the purity of the gangue material being 
analyzed. Accurate characterization of the original ore 



material amd the further feasibihty of smelting to ferro- 
chrome are identified by using this ratio. 

The method of analysis for silica at this Center is dehy- 
dration of the siUcic acid with sulfuric and perchloric acids. 
One dehydration is accurate enough for characterization of 
the ore. 



Equipment 

Zirconium crucible (approximately 30-mL capacity). 
Platinum crucible (approximately 20-mL capacity). 
Meker or similar gas-air mix burner. 
400-mL beaker and cover. 
Boiling or bump cup for 400-mL beaker. 
Glass stirring rod. 
Hotplate. 

Rubber policeman. 
Funnel. 

Filter paper and pulp. 

Fume cabinet or hood suitable for the fuming of 
perchloric acid. 
Muffle furnace. 



Materials 

Sodium peroxide, reagent grade, granular, 20 mesh or 
fmer. 

Hydrochloric acid, reagent grade, concentrated. 

Sulfuric acid, reagent grade, diluted 1:1 with distilled 
water. 

Perchloric acid, reagent grade, concentrated. 

Hydrofluoric acid, reagent grade, 48 to 51 pet. 

Hydrochloric acid, reagent grade, diluted 20:1 with 
distilled water. 



Procedure 



1. Weigh 0.5 g of the sample into a zirconium crucible. 

2. Add 4 to 12 g of sodium peroxide, and stir until the 
sample is thoroughly mixed. 

3. Fuse the crucible contents using the gas and air 
burner until all particles are dissolved completely. 

4. When the fusion is complete, allow the crucible amd 
its contents to cool. 

5. Gently tap the melt into a 400-mL beaker, and add 
50 to 75 mL of distilled water. 

6. Carefully add to the crucible 10 to 15 mL of distilled 
water and 5 mL of concentrated hydrochloric acid. When 
the chemical action ceases, slowly pour the contents of the 
crucible into the 400-mL beaker containing the mziin melt. 



7. Police the crucible, being careful to remove any ad- 
hering silica particles from the sides. 

8. Acidify the solution in the 400-mL beaker with con- 
centrated hydrochloric acid. 

9. To the beaker add 30 mL of 1:1 sulfuric acid and 
40 mL of concentrated perchloric acid. 

10. Place the beaker and its contents on a hotplate in a 
perchloric acid hood, and slowly evaporate the solution 
until dense white fumes of perchloric acid appear. 

11. Put a cover glass over the beaker, and reflux at a 
high-heat setting on the hotplate for 15 to 20 min. A 
boiling or bump cup made of aluminum or similar material 
helps avoid splattering. 



12. When the beaker is cool, add 150 mL of distilled 
water to the mass in the beaker and stir. SUghtly heat, if 
necessary, to dissolve any insoluble materiiJ other than the 
silica. 

13. Filter the solution through a medium-speed paper 
(such as S&S 589 white ribbon) plus a little paper pulp, 
and pohce the 400-mL beaker to remove any silica that 
adheres to the sides. 

14. Wash the sUica caught in the filter paper, alternately, 
five times with distilled water and five times with the 5-pct 
hydrochloric acid wash solution (heat sUghtly). 

15. After allowing the filter paper and its contents to 
drain properly, place them in a clean platinimi crucible. 

16. Starting at a low temperature in a muffle furnace, 
ignite the crucible and contents to 1,200° C for approxi- 
mately 1 h. 

17. Cool in a desiccator, and weigh the platinum crucible 
containing the ignited sihca and its associated impurities. 



13 



18. Add 3 to 5 mL of hydrofluoric acid and one to two 
drops of 1:1 sulfuric acid to the platinum crucible. 

19. Slowly volatilize the silica as sUicon tetrafluoride on 
a hotplate at a low heat. 

20. As soon as the hydrofluoric acid and silica have evap- 
orated and fumes of sulfvu"ic acid appear, increase the hot- 
plate temperature to a high setting. 

21. When drying is complete, ignite the platinum crucible 
and impurities to a red heat over a burner. 

22. Weight the platinum crucible and impurities when 
cool. 

Calculation: 

(wt Ft crucible + Si02+ impurities) - (wt Pt crucible + impurities) 



sample wt 



X 100 = pet total SiOj. 



Procedure Notes 



1. A sample size suitable for most chromite ores is 
0.5 g. If considerable (over 40 pet) sihca is present, reduce 
the sample size to 0.25 g. 

2. Increase the sodium peroxide as the Scunple size in- 
creases, at an approximate ratio of 20 parts of peroxide to 
1 part of sample. Again, however, consideration must be 
given to melt volume at high sample weights. Thorough 
mixing cannot be overstressed. Sample particles in the 
melt can aggregate and stubbornly resist attack. 

3. The burner should be capable of bringing the crucible 
to red heat on the bottom. Mixing remains important. If 
particles are allowed to aggregate, fusing time may have to 
be extended, which increases the attack on the crucible 
itself, shortening its useful life, increasing the tendency of 
the cooled melt to stick to the crucible, and adding a sig- 
nificant concentration of zirconium to the analyte solution. 

4. If the melt is to be leached quickly, the crucible can 
be set out on any heat-resistant material to cool. If the 
melt has to be set aside for a while, it should be set on a 
hotplate on low heat to keep it from absorbing atmo- 
spheric water (which makes the melt stick to the crucible). 

5. If gentle tapping does not free the melt, try mod- 
erately hard tapping. If the melt still sticks, lay the cru- 
cible on its side in the bottom of the beaker. 

6. The leaching reaction can range from very slow to 
very vigorous. A high aluminum concentration tends to 
slow the reaction. A slow leach reaction may become a 
very (ast leach reaction when sulfuric acid is added, so 



care must be exercised. Once the leach reaction has fin- 
ished, the addition of sulfuric acid is generally accom- 
panied by moderate effervescence unless a significant 
amount of carbonate is present. 

7. Using glassware will introduce negligible silica into 
the analysis, although one must be careful not to uninten- 
tionally add silica by accidentally chipping a stirring rod, 
etc. CfUefully pohce all glassware and crucibles to remove 
particles of sihca. Nonignited silica is quite sticky and ad- 
heres readily to most surfaces. 

9. The purpose of using sulfuric and perchloric acids is 
to dehydrate the soluble silicic acid Si02*H20 to insolu- 
ble sihca (SiOj). These two acids are excellent for this 
purpose. 

10. A speciail perchloric acid hood is needed for evapo- 
ration since most commercial hoods are not designed for 
this purpose and may react explosively to perchloric acid 
fumes. 

11. When the solution reaches a volume of approximately 
100 to 125 mL, salts start to precipitate. As this happens, 
occasional stirring keeps the solution well mixed and pre- 
vents bumping. A bump or boiling cup is a good preventa- 
tive measure against splattering. 

12. An acid volume of 50 to 75 mL is satisfactory for re- 
fluxing to begin. Do not fume off all of the perchloric 
acid. If this happens, insoluble chromium salts will appear, 
which will not redissolve upon adding water. Should this 
occur, add more perchloric acid and refume. Refluxing for 



14 



15 to 20 min removes almost all of the water from the 
silicic acid and also oxidizes the iron and chromium in 
solution to an orange color. 

13. After adding distilled water to the cooled and refluxed 
silicic acid, do not let the solution stand for long, as some 
of the colloidal silica will redissolve. It may be necessary 
to heat the beaker and its contents slightly or to add 
several milUliters of concentrated hydrochloric acid to 
redissolve any material that is difficult to dissolve. 

14. Wash the fdter paper carefully to remove all traces of 
perchloric acid. // this is not done, the remaining 
perchlorates will ignite explosively in the furnace. 

16. The ignition to 1,200° C effectively removes all traces 
of water from the silica. 

17. Ignited sihca (SiOj) is hght and fluffy. Avoid air 
drafts and spillage when weighing and igniting. The 
weighed silica is always impure, and with chromites it may 
have a light greenish color. Impurities include aluminum, 



chromium, iron, titanium, vanadium, and zirconium, as 
well as beryUium, calcium, amd strontium, if present in the 
initial material. The weight of impurities should be 
extremely small in compairison to the weight of silica 
obtained if the analysis is carefully done. 

18. Adding hydrofluoric acid to the weighed silica 
produces sihcon tetrafluoride (SiF^), which is a gas. This 
step must be done in a hood as fumes of hydrofluoric acid 
and silicon tetrafluoride are extremely toxic. Sulfuric acid 
also must be added to the platinum crucible containing the 
ignited sihca and impurities. When the platinum crucible 
is reignited with its residue of iron, aluminum, etc., 
everything remaining will be reignited to an oxide form, 
as originally done at 1,200° C rather than to a fluoride 
form. As an example, ferric sulfate [Fe2(S04)3] as an 
impurity in the platinum crucible is converted to ferric 
oxide (FejOj) and sulfur trioxide (SO3) upon reignition. 

21. Dirty platinum crucibles may be cleaned by fusing 
sodium bisulfate in them until the sides and bottom are 
free of stuck particles. 



MANGANESE IN MINERAL CHROMITE, FERROCHROME 
SLAGS, AND FERROCHROME 



Manganese commonly occurs in mineral chromite as an 
impurity, and the amount can range from trace quantities 
to a few percent. In ferrochrome smelting operations, the 
manganese should find its way into the slag rather than the 
metal. Data from these analyses have most often been 
used to trace the partitioning of the manganese through 
the smelting process and for material balance calculations. 

At this Center, the method of choice for slags 
and ores is decomposition by peroxide fusion, followed 
by a separation of interfering elements and oxidation of 

Equipment 

Zirconium crucible (approximately 30 mL capacity). 

Meker or similar gas-air mix burner. 

150-mL beaker. 

250-mL beaker cmd cover. 

Stirring rod. 

Magnetic stirrer and stirring bar. 

200-mL volumetric flask. 

100-mL volumetric flask. 

Rubber policeman. 

Funnel. 

Filter paper. 

Assorted pipettes. 

UV-visible spectrophotometer. 

Wash bottle. 



the manganese to permanganate, to be measured by 
molecular absorbance in an ultraviolet- (UV) visible 
spectrophotometer. 

MetaUic ferrochrome samples may be decomposed by 
fusion if the sample is ground to 100 mesh or finer and the 
sodium peroxide flux is moderated by the addition of 10 to 
20 pet of sodium carbonate. Acid digestion beginning with 
approximately 5-pct sulfuric acid and a trace of hydro- 
fluoric acid is the alternative. 



Materials 

Sodium peroxide, reagent grade, granular, 20 mesh or 
finer. 

Sodium carbonate, reagent grade, anhydrous powder. 

Sulfuric acid, reagent grade, diluted 1:1 with distilled 
water. 

Hydrogen peroxide, reagent grade, 30-pct solution. 

Zinc oxide, reagent grade, slurry. 

lodate oxidant solution. 

Standard manganese solution. 



15 



Procedure 



1. Weigh the sample into a zirconium crucible. 

2. Add 5 to 10 g of sodium peroxide (plus 1 to 2 g of 
sodiiun carbonate for ferrochrome metal), and stir until 
the sample and flux are thoroughly mixed. 

3. Fuse over a burner until all sample particles are 
dissolved. Swirl and inspect occasionally to keep unat- 
tacked particles dispersed. 

4. When the fusion is complete, allow the crucible and 
melt to cool. 

5. When they are cool, tap gently to free the melt from 
the bottom of the crucible. 

6. Place the soUdified melt in the 250-mL beaker, add 
about 20 mL of distilled water, and cover immediately. 

7. Place the crucible in front of its beaker, and add to 
the crucible about 5 mL of distilled water and about 2 mL 
of 1:1 sulfuric acid. 

8. Police the crucible thoroughly, and slowly rinse its 
contents into the beciker with a small amount of distilled 
water. 

9. Remove and rinse the beaker cover and the beaker 
sides. Place a stirring rod in the beaker. 

10. Slowly and with vigorous stirring, acidify the leach in 
the beaker with 1:1 sulfuric acid. 

11. Add 30-pct hydrogen peroxide dropwise with vigorous 
stirring to reduce the Cr** to Cr"^^ and bring the beaker to 
a boil to decompose any excess peroxide. 

12. When the solution is clear, remove from the hotplate 
and cool to room temperature. 



13. Place a stirring bar in the beaker and begin stirring at 
a moderate rate. 

14. Add zinc oxide slurry in small (3- to 5-mL) portions, 
allowing for dispersion between additions, until all iron 
and chromium are precipitated and a small excess of zinc 
oxide is apparent. 

15. Remove juid rinse the stirring bar, and transfer the 
sample slurry to a 200-mL volumetric flask. Use a wash 
bottle to ensure complete transfer. 

16. Cool to room temperature and make up to volume 
with distilled water. Stopper, mix, and let settle for a time. 

17. Using a dry funnel, dry paper, and a dry beaker, fil- 
ter a portion of the supernatant Uquid through a medium- 
speed qualitative paper (such as S&S 597). 

18. Pipette an ahquot of dry filtered solution into a 
150-mL beaker. 

19. Add 20 mL of the iodate oxidant solution, and set the 
beaker on a hotplate at low to medium heat. 

20. Leave the beaker on the hotplate for about 15 min 
after the first appearance of the permanganate purple. 

21. Remove the beaker from the hotplate, and cool to 
room temperature. 

22. Transfer the solution to a 100-mL volumetric flask, 
make to the mark with distilled water, stopper, and mix. 

23. Measure the absorbance of the solution with a 
UV-visible spectrophotometer at 545 nm. 

24. Prepare a cahbration curve by pipetting appropriate 
aliquots of a standard manganese solution into 150-mL 
beakers and following steps 19 through 23. 



Procedure Notes 



1. Sample size is estimated to yield 0.5 to 1 mg of 
manganese in the final aliquot for oxidation. 

2. Increase the sodium peroxide as the sample size 
increases, but consider the final melt volume when the 
sample weight approaches 1 g. When metcds are fused in 
sodium peroxide they tend to behave like a thermite 
mixture. Carbon will behave that way also but will not get 
as hot as a metal. The addition of sodium carbonate to 
the flux will slow the reaction of metal and peroxide. Iron 
burns very fast and very hot; chromium burns much more 
slowly. If a sample of ferrochrome were as high as 80 pet 
chromium, moderation with sodium carbonate would prob- 
ably not be necessary as long as the sample and flux 



were well mixed. Efficient mixing of the sample and flux 
cannot be overemphasized. Several milligrams of sample 
left unmixed can produce a spot hot enough to burn 
through a zirconium crucible. 

3. The burner should be capable of bringing the bottom 
of the crucible to red heat. When the flux and sample 
reach sintering temperature, sample particles may begin to 
burn. This produces reddish-orange flashes and some 
hissing and popping sounds. When this happens, remove 
the crucible from the flame and attempt to swirl the con- 
tents so that the heat of the burning metal will be 
absorbed by the remaining flux and sample mix. This is 
called autofusion and is common in samples with a very 



16 



high carbon content and well-moderated flux. Rarely will 
any of the sample-flux mixture be ejected from ine crucible 
if mixing has been thorough. Very rarely a sample-flux 
mix will "skyrocket" or be uncontrollably active. If this 
happens, while holding the crucible with tongs, remove the 
crucible from the flame and hold it as still as possible until 
the activity has subsided. Let the melt soUdify and drop 
the crucible into a large beaker or sink partly filled with 
tap water. Spilled melt may sometimes be chipped off 
such materials as transite and stone counters, but thorough 
cleanup will require the use of dilute acid and plenty of 
water. Skyrocketing is nearly always caused by inaccurate 
sample estimates, very poor mixing of sample and flux, or 
lack of flux moderation; with care it is avoidable. 

4. If the melt is to be leached soon, then the crucible 
may be placed on any heat-resistant material to cool. If 
the melt must sit for some time before leaching, the 
crucible can be placed on a low-heat hotplate to keep it 
from absorbing atmospheric moisture. 

5-6. If gentle tapping does not free the melt, then 
stronger tapping should be tried. If the melt sticks 
stubbornly, then place the crucible upside down on a clean 
piece of some durable, nonbrittle, nonreactive material 
(such as a clean scrap of counter stone). Tap on the 
bottom of the crucible with a light hammer until the melt 
is broken free. Transfer the melt to a 250-mL beaker, and 
rinse any small melt particles into the beaker also. 
Proceed with the leach. 

7-8. Often a small part of the melt will stick in the 
crucible bottom, and the melt will splatter on the crucible 
walls during fusion. These walls should all be poUced. 
Pour the policing solution into the beaker slowly and 
carefully. The solution in the beaker is very basic, and the 
solution in the crucible is moderately acidic. 

9. The leaching reaction is usually quite active and often 
splashes leachate on the cover and sides. 

10. Considerable heat is evolved when acidifying the 
highly basic leachate with the highly acidic 1:1 sulfuric acid. 
Excess sodium peroxide will also be partially decomposed 
and give off oxygen. Vigorous stirring and slow acid 
addition are therefore imperative. Observe closely during 
the acid addition to keep the reaction from boiling out of 
the beaker. If sodium carbonate was added to the flux, 
then carbon dioxide will be evolved on acidification, 
causing considerable foaming and requiring even closer 
attention. 

11. Ideally, just enough acid should be added to com- 
pletely dissolve the melt. Practically, attempt to keep the 



excess of acid small; it will be neutralized in a later step. 
The solutions resulting from fused samples usually have 
most, if not all, of their chromium in the f 6 state. It is 
necessary to reduce this chromium to the + 3 state so that 
it win be precipitated by the zinc oxide since the Cr^* will 
interfere in the absorbance measurement. The most often 
used method of reducing the Cr^^ to the Cr^^ state is to 
add small portions of 30-pct hydrogen peroxide until an 
addition produces no color change and little more effer- 
vescence. The solution is then boiled for several minutes 
to destroy excess peroxide. The addition of hydrogen per- 
oxide to cm acidic solution of Cr^* first produces the pur- 
ple Cr^^ ion, which then disproportionates to Cr^^ and 
Cr**. This purple color is very intense; lack of it, upon a 
peroxide addition, is the prime indicator of complete 
reduction. Boiling will decompose excess hydrogen per- 
oxide, but a small amount remains intact. This excess 
peroxide will consume the oxidizing solution by reacting 
with the oxidized manganese. Therefore, close attention 
must be given during the Cr^* reduction to minimize the 
hydrogen peroxide excess. 

12. The zinc oxide precipitation will cause heat to be 
evolved, so the solution should be cooled. 

13-15. Stirring should be vigorous enough to completely 
disperse the added zinc oxide slurry but not vigorous 
enough to cause bubbles. For the best results, allow the 
first few additions to dissolve completely before continuing. 
When the precipitate becomes dark and begins to persist, 
make the additions slightly larger and more frequent until 
the precipitate no longer darkens but begins to lighten in 
color. The precipitation is complete when there are white 
particles of zinc oxide visible in the precipitate and the liq- 
uid looks slightly milky if the precipitate is allowed to set- 
tle for a minute or two. 

16. When mixing the contents of the flask, shake vigor- 
ously enough to dislodge from the glass any precipitate 
that has caked while cooling. 

18. Take an aliquot to approximate a manganese content 
of 1 mg if possible, but use 50 mL as a maximum. 

20. All of the manganese present should be oxidized with- 
in 5 min of the first appearance of color; 15-min digestion 
allows a safety factor. 

23. Measurements should be made the same day as the 
oxidation step. Checks made at this Center have indicated 
that the color is stable for at least 24 h, except in the case 
of very high manganese concentrations (more than 3 mg of 
manganese per 100-mL flask). 



17 



ACID DIGESTION OF FERROCHROME METAL 



At times it may be necessary or desirable to dissolve a 
ferrochrome sample in acid rather than fusing it in sodium 
peroxide. Acid digestion methods are slow, but require 
little in the way of attention. Two alternative methods are 



described here. The A method is the basic method to use 
for most ferrochrome samples. The B method is for very 
stubborn samples. 



METHOD A 



Equipment 

250-mL beaker and cover. 
400-mL beaker and cover. 
Hotplate. 
Funnel (optional). 
Filter paper (optional). 
Fume hood. 



Materials 

Sulfuric acid, reagent grade, 5-pct solution. 
Hydrofluoric acid, reagent grade, concentrated. 



Procedure 



1. Weigh the sample into a 250-mL beaker. 

2. Add 100 mL of 5-pct sulfuric acid solution and 1 to 
2 mL of concentrated hydrofluoric acid. 

3. Place the beaker on a low- to medium-heat hotplate, 
and cover. 

4. When the sample has dissolved, remove the cover and 
take the solution to fumes of sulfuric acid; allow it to fume 
for a few minutes. 

5. Remove the beaker from the heat, cool, and dilute 
with distilled water. Warm if necessary to redissolve the 
salts. 



6a. If the sample is for a total chromium determination, 
transfer the solution to a 400-mL beaker and go to step 11 
of the total chromium method. 

6b. If the sample is for total iron determination, transfer 
the solution to a 400-mL beaker, add 3 to 5 g of am- 
monium chloride, and go to step 10 of the total iron 
method. 

6c. If the sample is for manganese determination, cool to 
room temperature or below and go to step 13 of the man- 
ganese method. 



Procedure Notes 



2. The acid digestion must be started gently. If the 
initial acid concentration is too high, the chromium in the 
ferrochrome will be passivated or become very unreactive. 
The low initial sulfuric acid concentration and the addition 
of the small amount of hydrofluoric acid are to avoid this 
passivation. 

3. The solution will slowly lose water; do not allow the 
total volume to drop below about 25 mL until all of the 
sample has dissolved. 



4. Do not take the solution to copious fumes or allow it 
to fume for more than a few minutes. Excess fuming can 
form refractory chromium salts that are as difficult to dis- 
solve as chromite itself. 

6. The solution may contain carbon or precipitated 
silica. To remove them, if desired, filter the solution 
through a medium- to slow-speed quaHtative paper and 
wash thoroughly. 



METHOD B 



Equipment 

250-mL beaker and cover. 
400-mL beaker and cover. 
Hotplate, 
Funnel (optional). 
Filter paper (optional). 
Perchloric acid fume hood. 



Materials 

Perchloric acid, reagent grade, concentrated. 
Nitric acid, reagent grade, concentrated. 



18 



Procedure 

1. Weigh the sample into a 250-mL beaker. 4. Cover the beaker and set on a medium- to high-heat 

hotplate. 

2. Add enough water to just cover the sample and 

25 mL of perchloric acid. 5. Bring to fumes of perchloric acid, and fume, covered, 

until all metal particles are dissolved. 

3. If the sample is known to have a high carbon content, 

add 20 mL of concentrated nitric acid. 6. See step 6 of method A. 

Procedure Notes 



3. Nitric acid is necessary only when carbon is present 
in percent amounts. The nitric acid partially oxidizes the 
carbon so that when the perchloric acid begins to react 
with it the carbon does not react explosively. Carbon that 
has not been predigested with nitric acid wiU often react 
explosively when the perchloric acid reaches fuming 
temperature. 

4-6. The sample is attacked by fuming perchloric acid to 
produce chromic acid. The process is slow, and the cover 
is necessary to provide some reflux. Chromic acid crystals 
are usually produced, and it may be necessary to cool the 
beaker and dissolve the crystals with water to see if 
the last particles of sample have dissolved. Silica is 
precipitated by the fuming perchloric acid and may be 



visible as hght-colored, low-density particles in the 
solution. Carbon is oxidized to carbon dioxide by the 
fuming acid, but occasionally graphite will survive the 
treatment. If graphite is present, it will usually be found 
floating on the surface of the solution and occasionally as 
dark-colored, low-density particles on the bottom. 
Unattacked sample will be the only heavy or high-density 
particles in the beaker. If no heavy particles can be seen, 
then the solution is considered complete. If the sample is 
to be used for a manganese determination, the chromium 
should be reduced to the +3 state. (See step 11 of the 
manganese method.) If removal of the perchloric acid is 
desired, this ccm be done by adding 20 mL of 1:1 sulfuric 
acid and taking it to fumes. 



DISCUSSION 



Analysts assigned to make a new determination on a 
sample often must deduce the significance of steps in the 
usually sketchy procedure descriptions of in-house methods 
and methods in journal or reference pubUcations. The 
familiau^ization process results in a delay in mastering the 
method. This report seeks to shorten that delay, allowing 
analysts newly assigned to the covered determinations a 
quicker understanding of the methods. 

The information presented here was collected empiri- 
cally by the analysis of thousands of samples over several 
years and the observation of the effects of small variations 
in technique. 



The rehance on zirconium crucibles as alkaline fusion 
vessels may have been noted. Their reUability in terms of 
durability and lowered introduction of interfering species 
in comparison to iron and nickel crucibles has been proven 
to the satisfaction of this Center. They should not, how- 
ever, be used in fusions in a muffle furnace because of sig- 
nificant oxygen attack. 

Any request for assistance concerning the subject of this 
report should be addressed to the authors, Bureau of 
Mines, Albany Reseau^ch Center, Albany, OR. 



REFERENCES 



1. Thomas, P. R., and E. H. Boyle, Jr. Chromium 
Availability-Market Economy Countries. A Minerals Availability 
Program Appraisal. BuMines IC 8977, 1984, 86 pp. 

2. Dahlin, D. C, L. L. Brown, and J. J. Kinney. Podiform Chromite 
Occurrences in the Caribou Mountain and Lower Kanuti River Areas, 
Central Alaska. Part II: Beneficiation. BuMines IC 8916, 1983, 15 pp. 

3. Kirby, D. E., D. R. George, and C. B. Daellenbach. Chromium 
Recovery From Nickel-Cobalt Laterite and Laterite Leach Residues. 
BuMines RI 8676, 1982, 22 pp. 

4. Cotton, F. A., and G. Wilkinson. Advanced Inorganic Chemistry. 
A Comprehensive Text. Interscience Publ., 3d ed, 1972, 1145 pp. 

5. Hillebrand, W. F., and G. E. F. Lundell. Applied Inorganic 
Analysis. With Sf>ecial Reference to the Analysis of Metals, Minerals, 
and Rocks. Wiley, 1953, 1034 pp. 

6. Jeffery, P. G. Chemical Methods of Rock Analysis. Pergamon, 
1975, 525 pp. 



7. Kolthoff, I. M., P. V. Elving, and E. B. Sandell (eds). Treatise on 
Analytical Chemistry. Interscience Encyclopedia, pt 2, v. 2-4, 7-8, 1961. 

8. Lundell, G. E. F., and J. I. Hoffman. Outlines of Chemical 
Analysis. Wiley, 1938, 250 pp. 

9. Lundell, G. E. F., J. I. Hoffman, and H. A. Bright. Chemical 
Analysis of Iron and Steel. Wiley, 1931, 641 pp. 

10. Scott, W. W., and N. H. Furman (eds). Scott's Standard Methods 
of Chemical Analysis. D. Van Nostrand, v. 1, 5th ed., 1948, 1234 pp. 

11. Weinig, A. J., and W. P. Schoder. Technical Methods of Ore 
Analysis of Chemist and Colleges. Wiley, 11th ed., 1948, 325 pp. 

12. Welcher, F. J. (ed). Standard Methods of Chemical Analysis. 
D. Van Nostrand, v. 1, 6th ed., 1962, 1401 pp. 

13. Young, R. S. Chemical Analysis in Extractive Metallurgy. 
Charies Griffin, 1971, 427 pp. 



19 



APPENDIX A.-REAGENT PREPARATION 



O.IN Fe*^ Solution 



Calculation 



Preparation 

Dissolve 39 g of ferrous ammonium sulfate hexahydrate 
in distilled water with 10 mL of concentrated sulfuric acid. 
Dilute to 1 L with distilled water. 

Standardization 

Weigh accurately 130 to 140 mg of primary standard- 
grade potassiiun dichromate into each of three 400-mL 
beakers. Add about 150 mL of distilled water and about 
5 mL of concentrated sulfuric acid, and stir until com- 
pletely dissolved. Titrate with the new ferrous solution to 
the disappearance of the diphenylamine sulfonate indicator 
purple. 



NCr^^ = 



Calculation 



A^Fe 



+ 2 wt K2Cr207 X pet purity K2Cr20-7 
meq wt K2Cr207 x 100 x mL titrated 

mg Cr A^ Fe^^ x 51.996 



mL Fe^^ 3 

1 mL O.IOOA^ Fe^' = 1.733 mg Cr. 
O.IN Cr*^ Solution 
Preparation 

Dissolve 4.9 g of potassium dichromate in distilled wa- 
ter and dilute to 1 L. 

Standardization 

Weigh accurately 0.17 to 0.18 g of electrolytic or NBS 
standard-grade iron metal into each of three 250-mL 
beakers. Add 20 mL of distilled water and 20 mL of 
concentrated hydrochloric acid, cover, and heat gently until 
the iron is completely dissolved. When dissolved, remove 
from the heat. Remove the cover, and rinse cuiy 
condensate into the beaker. Rinse the beaker sides, and 
quantitatively transfer the solution to a 500-mL Erlen- 
meyer flask. Make the volume up to 125 to 150 mL with 
distilled water. Bring to a boil and, dropwise, add 10-pct 
stannous chloride solution, with mixing, until the solution 
is colorless. Add 3 to 5 drops excess and just bring to a 
boil again. Cool to room temperature or below, and add 
10 mL of a saturated solution of mercuric chloride: mix, 
and add 10 mL of concentrated phosphoric acid. Titrate 
with the prepared Cr*^ solution using sodium diphenyl- 
amine sulfonate indicator to a purple endpoint. 



wt Fe X pet purity Fe 
meq wl Fe x 100 x mL titrated 

mg Fe 



A^ Cr"^^ X 55.847. 



mL Cr"^^ sol'n 
1 mL O.IOOON Cr"^^ = 5.585 mg Fe. 

Sodium Diphenylamine Sulfonate Indicator 

Dissolve 1.35 g of reagent-grade sodium diphenylamine 
sulfonate in 500 mL of distUled water. 

Vanadium-Acid Mixture for Ferrous Iron 

Add 3.0 g of vanadium pentoxide (V2O5) to a mixture 
of 300 mL of concentrated phosphoric acid and 600 mL of 
concentrated sulfuric acid. Heat and stir until all of the 
vanadium pentoxide has dissolved. Add, dropwse, a 
saturated solution of potassium permanganate until the 
acid mixture gives a faint pink when a few drops are 
diluted with water. Heat to fumes of sulfuric acid, and 
continue to fume for several minutes to destroy the excess 
permainganate. Cool thoroughly and transfer to a stock 
bottle. If filtration is necessary, use a pad of glass wool in 
a large funnel. 

Silver Nitrate Solution 

Dissolve 25 g of reagent-grade silver nitrate in distilled 
water and 10 mL of concentrated nitric acid. Dilute to 1 
L and store away from light. 

Stannous Chloride Reducing Solution 

Dissolve 20 g of stannous chloride in 200 mL of 20-pct 
(1:4) hydrochloric acid. Make this solution fresh weekly or 
store it in a bottle with several high-purity tin shot. 

Saturated Mercuric Chloride Solution 

Heat distilled water in a large beaker to almost boiling. 
Add reagent-grade mercuric chloride to the water by the 
spoon or spatula with vigorous stirring until no more will 
dissolve. Transfer to a stock bottle. 

Manganese Oxidant Solution 

Dissolve 7.5 g of reagent-grade potassium periodate in 
100 mL of hot 1:1 nitric acid. When dissolved, cool and 
add 400 mL of concentrated phosphoric acid, dilute to 1 L, 
and transfer to a stock bottle. 



20 



Saturated Manganese Solution 

Nearly fill a convenient size stock bottle with warm 
distilled water. Add manganese sulfate or potassium 
permanganate to the bottle, and shake or stir until 
dissolved. Repeat until no more will dissolve, and add a 
small excess. Transfer a portion of the supernatant liquid 
to a dropping bottle for use. Replenish the water or the 
manganese compoimd as necessary. 



Zinc Oxide Slurry 

Half fill a 250-mL beaker with zinc oxide powder. Add 
distilled water with stirring until the slurry is smooth and 
is the consistency of heavy cream or heavy latex pjiint. Stir 
the slurry occasionally diu-ing use.- 



21 



APPENDIX B.-USE OF ZIRCONIUM CRUCIBLES 



Zirconium crucibles are rapidly becoming a standiu-d 
piece of equipment, particularly in laboratories where 
fusions using strongly alkaline fluxes are common. This 
Center has found no better material to use for the thou- 
sands of sodium peroxide fusions that are done here every 
year. 

Zirconium crucibles usually arrive from their supphers 
in a polished, silvery state. They resemble a polished iron 
or nickel crucible very closely. If they become confused 
with iron or nickel crucibles, they can be identified by 
tapping them with a pencil or spatula and hstening to the 
tone produced. Zirconium crucibles produce a pleasant, 
almost musical tone when tapped. The first use of the 
crucible in a flame wiU produce a dark-gray coating over 
the metal that seems to enhance the crucible's resistance 
to sodium peroxide. It is thought that this coating is a 
nitride or a nitride-oxide mixture, but no attempt has been 
made to confirm or refute the idea. 

The use of zirconium crucibles should be limited to 
alkaline fluxes and fusions done over a burner. Zirconium 
crucibles should not be used for extended fusions or 
ignitions in a muffle furnace unless an atmosphere free of 
nitrogen and oxygen can be maintained in the furnace. 
The hottest part of the flame produced by a Fisher or 
similar design gas-air mix burner is proper for most 
sample-flux mixtures. If a high carbon, sulfide, or metaUic 
content sample is being fused, it is usually desirable to 
begin the fusion gently in case the sample goes into 
autofusion. The crucible may be supported on a wire 
triangle over the flame or held with tongs, depending on 
the analyst's preference. 

Autofusion occurs when the sample-flux reaction is 
exothermic, and the effect can range from barely de- 
tectable to explosive. Any sample known to contain a 
reduced species or a reducing agent should be fused in 
sodium peroxide moderated by the addition of anhydrous 
sodium carbonate, and extra care must be taken in as- 
suring complete mixing of the sample and flux. A well- 
mixed fusion will seldom eject material from the crucible 



even if the fusion becomes quite active. If an autofusion 
becomes too active or skyrockets, remove the crucible 
from over the burner as quickly as safety will allow. Place 
or hold the crucible with tongs where it will not contami- 
nate any other work, imtil the melt solidifies, and then 
drop the crucible into a large beaker or a sink partially 
fiUed with water. Allow the leaching reaction to subside 
and then retrieve the crucible, clean it, and check it for 
damage. 

With proper care, zirconium crucibles can last for more 
than 100 fusions. They should be checked before their 
first use for manufacturing flaws such as cracks or pits. 
During normal use, the bottom of the crucible is the part 
that receives the most severe abuse and the edge of the 
bottom is where most crucibles fail. As the crucible is 
used the bottom becomes thinner, and when it is thin 
enough so that it can be flexed with finger pressure, the 
crucible should be checked before each use for pinholes 
around the edge of the bottom. Discard any crucible that 
has a pinhole; the thin edge of a pinhole is particularly 
susceptible to attack by sodium peroxide. Also, do not use 
crucibles with thin bottoms to fuse samples that may auto- 
fuse. Extending the fusion time for a sample increases 
the attack on the crucible, which in turn shortens the life 
of the crucible and increases the amount of zirconium in- 
troduced into the sample. In general, the crucible and its 
contents will come to red heat within about 3 min. The 
melt in the crucible should then be swirled and the bottom 
examined for sample particles. The fusion is assumed 
complete when no more sample particles can be found. 
Some samples are not so easily distinguished, and only ex- 
perience with that sample type can form a basis forjudging 
completion. 

Sodium peroxide fusions should not be attempted out- 
side a fume hood. At the minimum, an operator should 
wear safety glasses and a lab coat or apron. Durable 
gloves would be advisable if samples known to be prone to 
autofusion are to be used. 



22 



APPENDIX C.-NOTES ON SAMPLE PREPARATION 



The grind size or mesh size of the sample is an ahnost 
crucial consideration in the analysis of mineral chromite, 
ferrochrome slags, and ferrochrome metal. In general, 
the finer the sample particle size, the easier it will be 
to dissolve. If all samples could be ground to pass 
a 2(X)-mesh screen, nearly all of the dissolution problems 
would be solved. In practice, a sample ground to pass a 
100-mesh screen will present little in the way of problems, 
but for mineral and slag samples in particular, larger 



sample particles should be avoided. Larger sample 
particles of metal, instead of being harder to fuse, are 
actually easier to fuse but have a tendency to produce hot 
spots in the crucible and so contribute to burnthrough. 
Larger metal particles, of course, are slower to dissolve in 
acid than smellier pcu^ticles. As a general rule, any sample 
ground to pass a 100-mesh screen should be acceptable, 
and any sample that will not pass a 60-mesh screen will 
present problems with sample decomposition. 



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